Converter Fundamentals James Bryant

1. Converter FundamentalsJames Bryant University of Leicester March 2003 Converter Fundamentals – Leicester U – March 2003

2. Converters Converter Fundamentals – Leicester U – March 2003

3. The Size of an LSB Converter Fundamentals – Leicester U – March 2003

4. Ideal Transfer Characteristics Converter Fundamentals – Leicester U – March 2003

5. Quantization Uncertainty Converter Fundamentals – Leicester U – March 2003

6. Unipolar & Bipolar Converters Converter Fundamentals – Leicester U – March 2003

7. Offset & Gain Error Converter Fundamentals – Leicester U – March 2003

8. Linearity Error Measurement Converter Fundamentals – Leicester U – March 2003

9. Differential Non-Linearity (DNL) Converter Fundamentals – Leicester U – March 2003

10. Combined Effects of Transition Noise & DNL Converter Fundamentals – Leicester U – March 2003

11. Sampled Data Systems Converter Fundamentals – Leicester U – March 2003

12. DAC Settling Time Converter Fundamentals – Leicester U – March 2003

13. DAC Transitions Converter Fundamentals – Leicester U – March 2003

14. Harmonic Distortion Converter Fundamentals – Leicester U – March 2003

15. Intermodulation Distortion Converter Fundamentals – Leicester U – March 2003

16. Third Order Intercept Point Converter Fundamentals – Leicester U – March 2003

17. Quantization Noise Converter Fundamentals – Leicester U – March 2003

18. Large Signal Bandwidth • With small signals, the bandwidth of a circuit is limited by its overall frequency response. • At high levels of signal, the slew rate of some stage (generally the output stage) may control the upper frequency limit. • In amplifiers, there are so many variables that “Large Signal Bandwidth” needs to be redefined in every individual case and “slew rate” is a more useful parameter for a data sheet. • In ADCs, the maximum signal swing is the ADC’s full-scale span, and is therefore defined so “Full Power Bandwidth may appear on the datasheet. • HOWEVER, the “Full Power Bandwidth” specification says nothing about distortion levels. ENOB is much more useful in practical applications • (If “Full Power Bandwidth” is specified and ENOB is not, somebody is probably trying to hide something!) Converter Fundamentals – Leicester U – March 2003

19. ENOB Converter Fundamentals – Leicester U – March 2003

20. SNR Due to Sampling Clock Jitter Converter Fundamentals – Leicester U – March 2003

21. Components for Data Converters • Data Converters require: • Good logic • Good switches • Good analog circuitry (amplifiers, comparators and references) • Good resistors Converter Fundamentals – Leicester U – March 2003

22. Hybrid Converters • Early Data Converters used hybrid technology to achieve performance unavailable from any single monolithic technology. • Even today, some of the best converters cannot use any available monolithic technology and are hybrid • “Compound Monolithic” is a marketer’s term for a simpler (and cheaper) hybrid technology where two monolithic chips from different technologies are mounted together in a single package, but without a ceramic substrate or other components. Converter Fundamentals – Leicester U – March 2003

23. Monolithic Converter Processes • Bipolar processes have good analog performance but less good logic and switches. • CMOS processes make excellent logic and switches but relatively poor amplifiers and lousy references. • Processes combining the two (BIMOS , LCCMOS, etc.) tend to be more complex and expensive and have slightly less performance than the sum of the two but are very convenient. • Good designers choose the best process for the circuit to be designed. Converter Fundamentals – Leicester U – March 2003

24. Thin Film Resistors • One of the key technologies for making many types of monolithic data converters is the ability to deposit accurate, stable SiCr resistors on monolithic chips. • Some converters use these resistors as fabricated; others require the additional accuracy and economy of laser trimming. • Parameters include matching to 0.005%, TC<20 ppm, Diff TC<0.2 ppm, and long term stability of the order of 1 ppm/1000 hours (drunkard’s walk). Converter Fundamentals – Leicester U – March 2003

25. Changeover Switches Converter Fundamentals – Leicester U – March 2003

26. Kelvin Dividers Converter Fundamentals – Leicester U – March 2003

27. Simplest Current OP DAC Converter Fundamentals – Leicester U – March 2003

28. Segmented Voltage DACs Converter Fundamentals – Leicester U – March 2003

29. Current Segment 4-Bit DAC Converter Fundamentals – Leicester U – March 2003

30. Binary Weighted DAC Converter Fundamentals – Leicester U – March 2003

31. DAC Using Cascaded Binary Quads Converter Fundamentals – Leicester U – March 2003

32. 4-Bit R-2R Ladder Network Converter Fundamentals – Leicester U – March 2003

33. Voltage-Mode Ladder Network DAC Converter Fundamentals – Leicester U – March 2003

34. Current-Mode Ladder Network DAC Converter Fundamentals – Leicester U – March 2003

35. Multiplying DACs (MDACs) • In all DACs, the output is the product of the reference voltage and the digital code. • Most DACs work only over a limited range of reference voltages • DACs which work with reference voltages which include zero volts are known as multiplying DACs • Many MDACs work with bipolar and AC references • DACs which work with a large range of reference voltages, but not down to zero, are not true MDACs but are sometimes called MDACs. It is better to use the term “semi-multiplying DACs.” Converter Fundamentals – Leicester U – March 2003

36. “Segmented Ladder” DAC Converter Fundamentals – Leicester U – March 2003

37. Audio DAC with Offset MSB Transition Converter Fundamentals – Leicester U – March 2003

38. Sigma-Delta DAC Converter Fundamentals – Leicester U – March 2003

39. Double-Buffered DAC Converter Fundamentals – Leicester U – March 2003

40. Serial DACs • If data is loaded serially into a DAC, it requires fewer data pins. • This saves space and also reduces capacitive noise coupling from data lines to the analog output . • If the shift register of a serial DAC has an output pin, a number of DACs may be connected in series (“daisy-chained”) to a single serial data port Converter Fundamentals – Leicester U – March 2003

41. Types of Analog-to-Digital Converters • Comparator: 1-bit ADC • Flash: Fast, low-resolution, power-hungry • Magamp: A new architecture with lower power and complexity but speed approaching that of a flash ADC • Subranging: Quite fast, high-resolution, complex • Integrating: Slow, accurate, low-power • VFC: High-resolution, low-power, ideal for telemetry • Tracking: Fast and slow, high-resolution • Successive Approximation: Versatile, general purpose • Sigma Delta: Complex, low-power, very accurate Converter Fundamentals – Leicester U – March 2003

42. Beware of ADC Logic Pitfalls! • After power-up, one or two conversions may be necessary before the ADC runs right. EOC cannot always be trusted at this time. An ADC may not behave the same way every time it starts. • EOC says conversion is finished. DRDY says that data is valid. There may be tens of nS difference between the two. • CS may not just enable the data--it may reset things for the next conversion. In some converters, you can’t not read the data. In some converters you can’t read the data twice. In some converters, you can’t strap CS and forget it. FIND OUT WHAT SORT YOU’RE USING. • ALWAYS READ THE DATASHEET, OR ELSE... Converter Fundamentals – Leicester U – March 2003

43. Comparators Converter Fundamentals – Leicester U – March 2003

44. Flash or Parallel ADCs Converter Fundamentals – Leicester U – March 2003

45. Flash ADC Input Model and Its Effect on ENOB Converter Fundamentals – Leicester U – March 2003

46. Mag Amps 1 Converter Fundamentals – Leicester U – March 2003

47. Mag Amps 1b Converter Fundamentals – Leicester U – March 2003

48. Mag Amps 2 Converter Fundamentals – Leicester U – March 2003

49. Mag Amps 3 Converter Fundamentals – Leicester U – March 2003

50. Mag Amps 4 Converter Fundamentals – Leicester U – March 2003